Control of a resonant wireless e-bike charging converter

Abstract

The Wireless Power Transfer (WPT) has been introduced decades ago for low power applications, and more recently, it has been used for industrial high power applications. WPT is gaining popularity because it presents several advantages over the power transfer through cable, such as the galvanic insulation between the power source and the load, the possibility of on-road charging for private electric vehicles, and it obviates the need of bringing around bulky cables of portable devices, especially the ones not standardized yet.

The subject of this thesis is an e-bike WPT charging system, using electromagnetic power transfer with resonant coupling. The WPT works through two coupled ferromagnetic coils with compensation capacitors. In particular, this thesis focuses on the control of the inverter of the e-bike WPT charging system.
Initially, some background on the WPT is given. Then, the problem treated in this project is defined, explaining the whole e-bike WPT charging system with its goals and constraints. The inductive and the resonant coupling are explained, including the possible compensation networks. Moreover, two different definitions of the bifurcation phenomenon are analyzed and compared. After this, power electronics topologies for both the primary and the secondary converters are presented, and the most suitable ones are chosen. Two possible operations for the inverter are discussed: the fixed frequency and the auto-resonant frequency operation. Additionally, a survey on the existing communication standards for WPT is presented, focusing in detail on the Qi specification from the Wireless Power Consortium.
The main contribution of this thesis is then explained in detail, which is the design of the control loop for the inverter, such that it works at auto-resonant frequency by automatically changing the operating point to achieve ZCS. The e-bike WPT charging system working at auto-resonant frequency gives a higher efficiency than when it is working at fixed frequency. Another crucial part of this thesis is also the validation of the theoretical model with measurements on a laboratory set-up. Finally, the main conclusions on the inverter control for the e-bike WPT charging system are given, together with recommendations for future research on the topic.